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Primus 2490 Unofficial Repair Guide
This is an unofficial picture based guide covering my own experience with repairing the Primus 2490 Central Heating System. The PCB Overlay picture (see the bookmarks) should give you a fair chance of repairing other parts of the circuit that are not mentioned here. The circuits are simple designs realised with the bare minimum of components necessary. They are fully serviceable by the average DIYer. The control unit for the boiler should not be powered up without some kind of spark plug connected. Doing this could damage the ionic flame sensing circuit. The ignition is a dual spark type, with the second spark occurring across a gap in the PCB inside the box.
The unit must be sealed tight to prevent gas from entering the box.
Please see the official manuals for general system wiring and system checking.
The Control Panel is mentioned later in this guide.
The lid is fastened by hooks and a sealant. It can be carefully pried off using a flat screwdriver. Do not pry against the PCB itself, as it is very fragile. The connector terminals and the ignition coil are a part of the casing. De-soldering these completely will allow you to lift out the PCB.
You may want to test the complete unit first before doing any further disassembling. The power source should supply 12V DC at minimum 1A and current limited to max. 2A. Wires can be soldered directly to the board according to the connection overview picture below.
Connection Overview
Boiler Thermostat Vcc Pump Gnd
Solenoid Hold Solenoid Pull Solenoid Gnd
Internal Spark Gap Ignition Coil Secondary #1 & Ionic Flame Sensor
Input Winding
Low V. Output Winding Ignition Coil Primary High V. Output Winding
Bridge the boiler-thermostat terminals during testing. If the ignition coil is connected, remember to connect an ordinary automotive spark plug. With the spark plug gap wide open, the spark should be able to fly to the ring of the plug. If the PCB is tested bare, a 10ohm wire-wound resistor can be used in place of the ignition coil primary winding. The current pulses are short but very high.
Basic Function
Ignition Capacitor
Q4 and VZ1 form a 7.5V regulator with a 470uF cap at the input and a 220uF cap at the output. This voltage is very noisy as it also feeds the switching high voltage converter. Q5 and Q6 are parts of an oscillator feeding Q7 followed by Q8 which drives the input winding on the transformer. The low voltage output from the transformer is rectified by D5, and used to charge (via a resistor) the 47uF/63V cap close to Q12/Q14 during the first few seconds (set by C14) of each power cycle. This charge is used to energize the 12V hold-relay (to the right) when its driver transistor opens. Once energized the relay is held by 7.5V via one of its contacts and a resistor. The hold-relay feeds the 5V pull-relay (to the left), which is turned off after successful ignition. This way the gas-solenoid cannot open unless the voltage converter is running. The high voltage output from the transformer is rectified by D8, and used to charge the ignitioncapacitor via the resistor close to S8. The voltage across the ignition-cap should reach about 200V during the first few seconds of each power cycle. Once the relays energize, the ignition-cap is shorted to ground about twice every second by the SCR Q18 for a set period of time. The SCR is triggered when the voltage across C11 reaches the threshold of VR2. Q16 and Q17 are probably involved in the firing sequence. The 270k resistor charging C11 was defective in my unit and was replaced by one with a higher voltage rating.
The two large blue resistors together with Q11 form a current path for the ionic flame sensor. The ionised gas in the flame is electrically conductive. If this current is not present at the end of the firing sequence, or disappears during normal operation, the hold-relay will turn off, the "emty" light on the control-panel will light, and the whole unit will need to be power-cycled to start a new sequence. Q2 and Q3 are probably involved in latching the "emty" light. After the ignition sequence the oscillator goes into idle with a shorter pulse length, resulting in about 140V across the ignition cap. The idle voltage drives the ionic flame sensor.
PCB Overlay
In this picture the track-side image is made transparent and placed on top of a mirrored image of the component-side. This makes it easier to see how the components are interconnected.
Replacement Transformer
In my unit the transformer was defective. A replacement transformer can be made by adding a custom input winding to a 1.6VA 230V/2x18V toroidal transformer. The winding halves are used in series. I used a 2x22V transformer, but ended up having to put a zener-diode across the 47uF/63V cap that energizes the holdrelay to limit the voltage. 2x18V should be better, but remember to check the voltage. Drill the centre-hole larger (8-9mm) with great care and chamfer the edge of the hole generously. Add 2x80 turns (about 2x5m) of 0.3-0.4mm enamelled copper wire and connect the windings in series. You will probably have to remove some turns to get the desired 200-220V across the ignition cap. I seated the transformer gently in a padded machine-vice, laid the two wires on the floor, and walked back and forth 80 times, bending the wires through the hole with a Y-slotted stick. Wind the wires reasonably tight to avoid too much noise. Connect the two 18V windings in series. Add a 1N400X clamping diode from Q8 to gnd on the PCB to clamp the high negative voltage that the transformer induces back into the oscillator. ( I placed mine in front of the yellow tantalum cap C14.) In this picture the transformer is elevated above the surrounding components by a couple of rubber foam pads. The flat headed screw has tape wrapped around it to centre the transformer and protect the new winding. The tracks around the flat screw head have been cut. 3 new ground holes and 3 new holes for the windings have been drilled in appropriate places. The input winding goes to Q8. The low voltage output winding goes to D5. The high voltage output winding goes to D8.
Do not get the output windings mixed up!
The output windings are sensitive to orientation. Settle for one orientation of the input winding, and try which orientation of the output windings that yields the highest DC voltage across the ignition capacitor and the 47uF relay capacitor respectively. All the electrolytics have been changed to good quality long life Japanese types. Changing the 4 largest caps would probably have been sufficient. Q8 is doing a heavy job and has been changed to a BD437. The relay contacts were difficult to inspect, so the relays were changed as well.
Control Panel
The temperature regulation in my unit was unstable. The potentiometer is a 1k linear type. It was changed to a long life conductive plastic type from TT Electronics (P160KN-0QD15B1K). This pot can be used with the original knob. DigiKey 987-1309-ND
One of the switches was worn, so all four were replaced with C&K F2UEE. DigiKey 401-1223-ND One of the red LEDs was defective and was replaced. All the electrolytics were changed to good quality long life types.
The circuit is based on two quad OPs type LM324 and a regulator.
There is also an older version of the circuit. I found this picture on the internet.
Keep up the DIY spirit! Best wishes, Simon.